The ink-jet printing process is a known duplicating technology in which the printing ink is transferred without pressure, that is, without contact of the print head with the print medium. Drops of ink whose deflection can be controlled electronically are sprayed out of a jet thereby onto a receiving material. This technology, which is also designated as pressureless printing, is particularly suited for printing products with irregular surfaces and packaging because there is a certain distance between the print head and the material to be printed. The printing process is very flexible. It is relatively economical and is therefore also used in computer printing, e.g., as a workplace printer.
The ink-jet method is also being increasingly used in industry, e.g., for outdoor advertising. In outdoor advertising the ink must meet special requirements as concerns the light-fastness and water resistance. In addition, the chromophoric inking substances must be very fine so that the printing jets do not become clogged. Both dyes and pigments are used as chromophoric substances. The latter have the advantage over dyes that the light-fastness is very high and that they are water-resistant. U.S. Pat. No. 5,085,698 offers a survey of the pigments and dyes used in ink-jet inks.
Pigments have the advantage over water-soluble dyes that they form stable dispersions with a high storage life only on the basis of a treatment with surfactants. According to the current state of the art ink-jet inks are only conditionally water-resistant and can therefore not be readily used in outdoor advertising.
Suggestions have been made for improving the water resistance of ink-jet inks. Thus, ionic or non-ionic surfactants are used for the stabilization of pigments in a dispersion. To this end water-soluble polymers can be used whose concentration is a maximum of 10% relative to the final formulation of the inkjet ink (see EP 0,781,819 A2).
WO 95/04109 describes the use of cyclopolymers for pigment stabilization. Residue groups thereof include a hydrophilic and a hydrophobic group. The hydrophobic group contributes to improvement of the water resistance by means of the water-repellent action. However, polymers have the disadvantage that they sharply raise the viscosity of the ink-jet ink. However, ink-jet inks must have a very low viscosity in order that the ink remains capable of being sprayed. Thus, the used of polymers is very limited.
WO 97/47698 describes a surface modification of pigment black with silanized, hydrophobic silicic acids for improving the water resistance. The silicic acid is sprayed into the pigment black reactor thereby. The retort temperatures are approximately 1800.degree. C., as a result of which the hydrophobing agent is thermally decomposed and loses much of its effect.
U.S. Pat. No. 5,618,338 describes a reactive two-component ink and a corresponding print head. The components, color plus cationic substance, react to provide an ink with an improved water resistance. The lamination of non-water-resistant inks is widespread. The disadvantage of the last-named, known methods is that they are very expensive.
There is therefore the problem of improving the water resistance of ink-jet inks while avoiding the disadvantages of the state of the art.